The invention relates to a photomask comprising a base plate of a radiation-transmitting and electrically insulating material, a first side of which is provided with a layer of an electroconductive mask material wherein, inside a closed edge of that material, a mask pattern to be imaged is formed, the photomask being entirely enveloped in a protective layer of a radiation-transmitting, electroconductive material.
Such a photomask can particularly suitably be used in the manufacture of semiconductor devices and flat panel displays. In practice, the base plate generally is a plate of quartz glass, the layer of mask material generally contains chromium. The pattern of the mask is imaged, often on a reduced scale, on a layer of a photoresist which is provided, for example, on a layer of a metal. After exposure and development of the photoresist, the layer of metal can be etched in a pattern of conductor tracks corresponding to the mask pattern. The photomask pattern to be imaged comprises many closely spaced tracks of a mask material which are sometimes connected to larger areas of a mask material. The tracks correspond to the conductor tracks to be formed in the layer of metal, and the areas correspond to, for example, bond pads to be formed in the metal layer, which bond pads are used for contacting the devices to be manufactured.
The photomask is entirely enveloped in a protective layer of radiation-transmitting, electroconductive material. As a result, the mask pattern is incorporated in a Faraday cage which protects it against damage that might be caused by electrostatic discharges (Electro-Static-Discharge) in the mask pattern. When use is made of a photomask which does not have such a protective layer, in practice, the mask material present on the electrically insulating base plate may become electrically charged. This may be caused, for example, by air flowing past or by friction brought about by contact with clothing and other insulating materials. In order to be able to image the photomask onto the layer of photoresist, said photomask is placed in a projection apparatus on a grounded mask holder. In this manner, the closed edge of mask material present around the mask pattern is grounded. Voltage differences between this edge and the mask pattern may then develop, the magnitude of which is such that electric discharges occur in the mask pattern, which lead to said damage.
In JP-A-57-60335 a description is given of a photomask of the type mentioned in the opening paragraph, wherein the protective layer of radiation-transmitting, electroconductive material, in which the mask is enveloped, is a layer of indium oxide. The protective layer is deposited on the mask pattern on the first side of the mask and is situated on and between tracks of mask material.
The protective layer, which is deposited from the vapor phase, exhibits a homogeneous thickness. On the edges of the tracks of mask material extending transversely to the base plate, the thickness of the protective layer is substantially the same as on and between the mask tracks. Viewed transversely to the base plate, however, the thickness of the protective layer right next to the mask tracks is larger than centrally between the mask tracks. Consequently, right next to the mask tracks, radiation going through the mask during the projection of the mask pattern onto the layer of photoresist, passes through a much thicker layer than centrally between the mask tracks. Since such a layer has a refractive index which differs from that of air, diffraction phenomena will occur when the photomask is imaged. This may cause errors upon imaging the mask tracks onto the layer of photoresist.